An optical detection method using flow cytometry (flow cytometer) is used for identifying biological microparticles such as cells, microorganisms, and liposomes. Flow cytometry is an analysis method of identifying a plurality of particles one by one by illuminating particles flowing in a line through a flow path with laser light of a predetermined wavelength, and detecting fluorescence or scattered light emitted from each particle.
When target particles are cells, for example, by detecting forward scattered light (forward scatter: FS), information on the shape, size, surface state, formation of or presence or absence of a nucleus, or the like can be acquired. Forward scattered light from a target particle is generally detected by a light detector such as a photodiode, where light other than forward scattered light such as illumination light (hereinafter, also referred to as zero-order light) also enters.
As a technology for reducing the effect of light other than target scattered light, for example, there is a light scattering type particle detector having a light-receiving means formed with a plurality of photoelectric elements, and provided with an addition processing means for adding outputs of these photoelectric elements (see Patent Document 1). There is also a technology for removing unnecessary light by disposing a light-shielding mask on the optical path of scattered light. In addition, a microparticle measurement apparatus in which an optical filter is disposed in place of a light-shielding mask to improve the efficiency of detecting scattered light has also been proposed (see Patent Document 2).